|Publication number||US8072222 B2|
|Application number||US 12/059,511|
|Publication date||Dec 6, 2011|
|Filing date||Mar 31, 2008|
|Priority date||Mar 31, 2008|
|Also published as||US20090243614, WO2009146005A2, WO2009146005A3|
|Publication number||059511, 12059511, US 8072222 B2, US 8072222B2, US-B2-8072222, US8072222 B2, US8072222B2|
|Inventors||Leendert Combee, Michael Malling, Phil Heelan|
|Original Assignee||Westerngeco L. L. C.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (9), Non-Patent Citations (5), Referenced by (1), Classifications (9), Legal Events (2)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The invention relates to providing a signal generator to produce a signal having an analog continuous waveform for use in surveying a subterranean structure.
Electromagnetic techniques exist to perform surveys of subterranean structures for identifying layers of interest. Examples of layers of interest in the subterranean structure include subsurface resistive bodies, such as hydrocarbon-bearing reservoirs, gas injection zones, thin carbonate or salt layers, and fresh-water aquifers. One type of electromagnetic (EM) survey technique is the controlled source electromagnetic (CSEM) survey technique, in which an electromagnetic transmitter, called a “source,” is used to generate electromagnetic signals. Surveying units, called “receivers,” are deployed on a surface (such as at the sea floor or on land) within an area of interest to make measurements from which information about the subterranean structure can be derived. The receivers may include a number of sensing elements for detecting any combination of electric fields, electric currents, and/or magnetic fields.
An EM source produces an EM field by driving a controlled electrical signal that is provided through an electrode of the EM source. The signal produces an EM field that is emitted through an antenna of the EM source.
Traditionally, the electrical signal that is generated by the EM source is a switch-on/switch-off signal, where the electrical signal when in the switched-on state has a relatively large positive current amplitude, and when in the switched-off state, has either a zero or negative current amplitude. Generally, this switched-on/switched-off electrical signal is in the form of a square wave with abrupt steps (step up or step down). However, using a square wave signal to produce EM fields from an EM source limits flexibility and/or accuracy in performing EM surveying of subterranean structures.
In general, according to an embodiment, a system for performing electromagnetic (EM) surveying of a subterranean structure includes a signal generator to produce a signal having an analog continuous waveform without steps in the waveform. An antenna is responsive to the signals having the analog continuous waveform to emit an EM field to perform the EM surveying of the subterranean structure.
Other or alternative features will become apparent from the following description, from the drawings, and from the claims.
In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments are possible.
In an electromagnetic (EM) survey arrangement, an EM source according to some embodiments is used that has a signal generator for producing an analog continuous waveform that is provided to an antenna of the EM source. The antenna is responsive to the signal having the analog continuous waveform to emit an EM field to perform EM surveying of a subterranean structure. A signal having an analog continuous waveform refers to a signal that is without steps (step up or step down) in the waveform, as would normally be present in a square waveform (where the square waveform has an “on” state associated with a high positive current amplitude, and an “off” state associated with a low current amplitude, where the low current amplitude is either zero or negative).
The signal having the analog continuous waveform has amplitude levels (current amplitude levels or voltage amplitude levels) that vary substantially continuously over time such that amplitude steps are not present. The change in amplitude levels in the analog continuous waveform occurs over a time duration that corresponds to a frequency range of interest associated with an EM survey operation. This is contrasted to a square wave, where the abrupt step up or step down in amplitudes occur over time durations corresponding to frequencies that are outside the range of interest for performing the EM survey operation. In one example, the frequency range of interest is between 0.1 Hz (Hertz) and 10 Hz.
The use of a signal having an analog continuous waveform enables flexibility in selecting an EM source signal with optimized characteristics. For example, a survey operator can select a frequency (or plural frequencies) that are to be present in the analog continuous waveform signal to achieve a desired goal. Examples of enhanced characteristics include increased energy in a primary signal component (e.g., first harmonic) and/or more optimal spreading of harmonic signals at the desired frequencies.
Examples of the signal having an analog continuous waveform include one or more of the following. The signal having an analog continuous waveform can be a simple sine wave signal with a single frequency, such as signal 102 in
Another example of an analog continuous waveform signal is a sweep signal that has plural sine wave components having plural different frequencies, where the sweep signal is produced by scanning across the plural frequencies. An example of a sweep signal is represented as 104 in
Another example of an analog continuous waveform signal is a composite waveform signal 112 depicted in
EM surveying performed according to some embodiments with signal generators that produce signals having analog continuous waveforms can be provided in any of a land-based EM survey context, a marine EM survey context, or a wellbore EM survey context.
An example marine survey arrangement is depicted in
The marine vessel 200 is provided at a water surface 206 and the EM source 202 is towed in a body of water 208 underneath the water surface 206. Note that the EM source 102 can include a tow fish for navigating the EM source 202 in the body of water 208. Moreover, as depicted in
A main controller 222 on the marine vessel 200 has a power supply 220 that provides a high-voltage, low-current main power supply signal over an electrical medium in the tow cable 204 to the EM source 202. The EM source converts this high-voltage, low-current power supply signal into an analog continuous waveform signal that is provided to an antenna 230 of the EM source 202. In some embodiments, the antennas 230 can be an electric dipole antenna having electrodes 232 and 234. In other embodiments, other types of antennas can be used, including magnetic dipole antennas or non-dipole antennas.
The EM source 202 emits EM signals that are propagated into the body of water 208 and into the subterranean structure 214. Portions of the EM signals are reflected by the resistive body 216, and the reflected EM signals are detected by the EM receiver 210. Data corresponding to the detected reflected EM signals are stored in the EM receiver 210.
Subsequently, after some amount of time, the EM receiver 210 can be retrieved from the sea floor 212, at which point EM measurements made by the EM receiver 210 can be retrieved from a storage in the EM receiver 210. The retrieved EM measurements are subjected to processing to characterize the subterranean structure 214. Processing can be performed by the main controller 222 or by a remote computer located on a sea platform or on land.
In response to the commands/indications received from the main control module 304, the source control module 306 of the EM source 202 controls the signal generator 302 to cause the signal generator 302 to produce an analog continuous waveform signal (310) that is provided to the antenna 230 of the EM source 202. The antenna 230 responds to the analog continuous waveform signal by emitting an EM field propagated toward the subterranean structure to be surveyed.
The control module 306 of the EM source 202 then controls (at 406) the signal generator 302 to produce the analog continuous waveform signal according to the requested characteristic specified by the indication at 404. An EM field is then emitted (at 408) by the antenna 230 of the EM source 202 in response to the analog continuous waveform signal.
While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover such modifications and variations as fall within the true spirit and scope of the invention.
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|Citing Patent||Filing date||Publication date||Applicant||Title|
|US20140239958 *||May 6, 2014||Aug 28, 2014||Pgs Geophysical As||Methods and apparatus for adaptive source electromagnetic surveying|
|U.S. Classification||324/365, 324/357, 324/337|
|International Classification||G01V3/08, G01V3/12|
|Cooperative Classification||G01V3/12, G01V3/083|
|European Classification||G01V3/12, G01V3/08F|
|Mar 31, 2008||AS||Assignment|
Owner name: WESTERNGECO L. L. C., TEXAS
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:COMBEE, LEENDERT;MALLING, MICHAEL;HEELAN, PHIL;REEL/FRAME:020729/0393
Effective date: 20080325
|May 20, 2015||FPAY||Fee payment|
Year of fee payment: 4